The present disclosure relates to pavement systems that are suitable for use on weak subgrade, or native soil, or expansive clays, or soils susceptible to frost heaving during cold seasons. These pavement systems are located over the subgrade, and are used in various applications, such as roads, parkways, walkways, and railways. These pavement systems are especially suited for weak subgrades.
In transport engineering, several layers are recognized in the construction of a pavement. These layers include the subgrade layer, the sub-base layer, the base layer, and the surface layer. The subgrade layer is the native material and acts as the foundation for the pavement. The optional sub-base layer is laid over the subgrade. The sub-base and base layers are used to carry load and dissipate it to a level acceptable for the surface layer. Depending on the desired use of the pavement, another layer can be placed over the base layer, and this layer may be known as a paver base layer. The surface layer is then placed on top of this, and is the exposed layer on the surface of the pavement. The surface layer can be, for example, asphalt (e.g. a road or parking lot), or concrete (e.g. a sidewalk), or ballast (e.g. upon which railway rails are then laid), or compacted granular material (unpaved road).
A weak subgrade is a subgrade that has a California Bearing Ratio (CBR) of 4 or lower, or more typically 3 or lower, when measured when saturated with water. Weak subgrades have low stiffness and low resistance to load. Specific weak subgrades include those where the subgrade is an expansive clay or soil susceptible to frost-heaving during cold seasons. Frost heaving is an upwards swelling of soil caused by the formation of ice below the surface. The presence of water causes a few processes to occur that can be very damaging to pavements. First, the water molecules can swell the soil particles and lower the cohesion between them. Second, swelling by water can cause expansion of the soil, increasing pressure upwards on the pavement above. Third, the water expands during freezing, and in combination with hardening due to ice formation, can damage the pavement. These upwards stresses generated during expansion (e.g. swelling of the clay or the soil) can be significantly greater than those generated by traffic on soft subgrades. Pavements that are installed on such weak subgrade can fail prematurely.
In many situations where the subgrade is weak, and the subgrade is shallow, the subgrade is removed and replaced with stronger and more dimensionally stable granular materials. However, in other situations this is impossible due to: (a) the soft soil of the subgrade being too deep; or (b) stronger and more dimensionally stable granular materials not being available locally, or the cost of shipment of such materials being too high. Examples of these situations can be found in peat ponds in northern Russia, expansive clay beds in Texas, and muskeg beds in Canada and Siberia.
An example of a pavement is shown in FIG. 1. The pavement here includes a weak subgrade 2, a crushed stone base 4, and a surface layer 6. Again, the weak subgrade can be due to soft soil, expansive clay, or frost-susceptible soil. Typical failures include rutting (formation of a groove or rut in the pavement), cracking in the asphalt or concrete surface layer of the pavement, distortion or misalignment of railway rails laid on ballast, and pumping out of the base layer underneath the surface layer. These failure modes are caused by irreversible deformations to the base and/or sub-base due to the lack of (1) tensile strength; (2) stiffness (modulus); (3) interfacial strength between layer and subgrade; and/or (4) bending moment (resistance to bending).
One method commonly employed to prevent these failure modes includes the chemical modification of the subgrade. The subgrade is mixed with an inorganic binder (e.g. lime, cement or fly ash) or an organic binder (e.g. a polymer emulsion). However, this method is subject to several undesirable characteristics such as: slow curing, poor performance when applied in wet and cold climates, leaching of inorganic binders in wet climate, high cost of polymeric binders, brittleness, poor quality due to difficulty in field mixing, poor resistance to freeze-thaw cycles, and difficulty in obtaining a homogeneous subgrade over large areas (e.g. in texture or composition).
It would be desirable to provide pavement systems that have improved performance when installed over a weak subgrade, or native soils, or expansive clays, or frost-susceptible soils. It would also be desirable for such pavement systems to be constructed in an economical and easy to install method.